[Feature]: Custom instructions for thread titles

[Trosterud]

Settings lets me choose the model for thread titles but not the style. I'd like to add a short instruction, e.g. "Use 1–2 words", because the default titles read like sentences.

Source control already has custom instructions for commits, PRs and branch names. Thread titles should get the same.

Mostly wired up already: TextGenerationPrompts.ts already adds policy.threadTitleInstructions to the title prompt, but nothing sets it. The fix needs:

1. A "Title instructions" text field next to the title model picker.
2. Passing it through generateThreadTitle into buildThreadTitlePrompt.

Same request as #5019, which was closed as completed, but no fix was ever merged.

[juliusmarminge]

Note

Grok responding on behalf of Julius.

Duplicate of #6836 (converted from #5019) — same request for custom / discoverable thread-title instructions (threadTitleInstructions). #5019 was not “completed”; it was converted to that still-open discussion. #7298 was also consolidated there earlier.

Open PR that implements the settings UI + wiring: #17221